Speak to Praetor Magnus Auluxus in the Combine encampment in the southwest corner of Skyfire Mountains after doing the above quests.

Praetor Magnus Auluxus nods, 'Fortunately, we have the element of surprise. Our indagators are prepared to teleport you into a relatively quiet part of the ruling quarter. Are you bringing a [large] or [small] force?'

Praetor Magnus Auluxus beckons to you, magic in the air around him. 'If you are done, let's [disappear].'

Quest Window Says: The leaders of Gorowyn have bound their souls together into a grand phylactery. They will return as powerful ghosts soon after their death, unless they are all killed at the same time. You have been asked to put an end to them for good.